I've been reading Gideon the Cutpurse recently. It's a decently-well-written story about two kids in modern England who accidentally travel back to the mid-18th-Century. (Think The Golden Compass with somewhat less imaginative world building.)

The word in question is "bottom." As used by those native to the 18th Century in England, it means something like "courage" or "gumption." From what I can tell, the now-common meaning "derriere" did not come about until nearly the end of the 18th Century. The problem I'm having is that I cannot figure out how the "courage" meaning came about for a word that has its roots strongly in "base," "fundamental," or "ground." (It also seems a bit odd that whoever coined "bottom" as "derriere" clearly thought of the rear-end, and not the feet as the "bottom" of the human. I guess he sat a lot.)